Our objective is to characterize pathways whereby intravenous (IV) lipid emulsions, commonly used clinically as nutritional support, deliver specific types of triglyceride (TG) to tissues and cells. With increasing recognition of the importance of omega-3 (omega-3) very long chain fatty acids (FA) (omega3-VLC FA) in multiple biological pathways, our major goal is to delineate mechanisms for delivery of these omega3 FA to different tissues, and to define the effects of this delivery on selected cellular endpoints related to cell TG and cholesterol metabolism. Recent evidence indicates that routes for blood clearance and tissue uptake of fish oil omega3 TG are very different from those of omega6 soy oil long chain TG (LCT). For example, removal of (omega3 VLCT) emulsions from blood seems to depend far less on intravascular lipolysis than does LCT emulsions. While substantial amounts of both emulsions are delivered to tissues as intact TG, this pathway is likely more important for omega3 TG particles. Omega3 TG particles are less dependent on "classical" lipoprotein receptor related clearance pathways, than are LCT. FA derived from omega3 TG appear to act as stronger inhibitors than LCT in sterol regulatory element (SRE) dependent gene expression-genes that are involved in both TG and cholesterol syntheses. The proposed research will systematically define mechanisms whereby TG composition, particle size, and specific ligand related pathways contribute to LCT vs. omega3-VLCT clearance and metabolism in different tissues and cell types. Aim 1 will use physical-chemical approaches (e.g., 13C-NMR) to clarify how omega3-TG and FA affect the properties of the emulsion particle surface, and how this in turn affects lipoprotein lipase and apoprotein E binding, structure, and function. Aim 2 utilizes in vitro cell culture experiments and Aim 3 in vivo mouse models to differentiate metabolic pathways for omega3-VLCT/FA and omega6 TG/FA, and how these differences affect expression of genes related to TG and cholesterol homeostasis. Studies in Aim 4 will test if hypotheses generated from cell and mouse models are likely to occur in humans -- by protocols whereby omega3-VLCT and LCT emulsions are infused in healthy volunteers. Our analyses will differentiate between mechanisms for clearance and metabolism omega3 vs. omega6 TG and FA. Our studies will allow new insight as to how fish oil derived emulsions reach different tissue, undergo metabolic processing, and regulate expression of certain genes, and will also provide carefully characterized models for the study of human lipid and lipoprotein metabolism.